Magnetism and Compass

Earth’s Magnetism

Magnetism

• Magnetism is the property of certain materials to create a magnetic force
• Magnetic force attracts or repels similar materials
• Ferrous metals like steel, nickel or cobalt can be magnetised
• Non-ferrous metals like aluminium cannot be magnetised

Magnetic Field

• Magnetic field is the space around magnet where magnetic influence is felt
• Magnetic lines of force is pattern in a magnetic field
• Varies with shape and strength of magnet
• Red end is the north seeking end which points to magnetic north
• Blue end is south seeking end which points to magnetic south
• Like poles repel whereas unlike poles attract each other

Magnetisation

• A metal can be magnetised by three methods
• Moving magnet repeatedly over a metal called stroking
• Hammering magnetic material with a magnet
• Applying direct current in a coil of wire wound around magnetic material

De-Magnetisation

• A magnetised metal can be de-magnetised by three methods
• Hammering magnet by placing it at right angles to earth magnetic field
• Heating the magnet above 900 deg c
• Applying alternating current and reducing current slowly
• Polarity reversal removes magnetism

Hard and Soft Iron

• Hard iron are metals such as cobalt and tungsten steel
• Hard iron is difficult to magnetise but retains magnetism longer
• Hard iron are called permanent magnets
• Soft iron are metals such as silicon and pure iron
• Soft iron are easy to magnetise but retains magnetism temporarily
• Soft iron remains magnetised only till passage of current in solenoid

Earth Magnetic Field

• Magnetic compass is the primary direction finding instrument in an aircraft
• Earth’s magnetic field is not exactly aligned to its polar axis
• Magnetic north is not co located with True north
• Magnetic north is approximately 400 NM away from True north
• Magnetic north moves at 22 NM per year
• Magnetic North and South poles are not diametrically opposite
• Magnetic compass uses only the North seeking end for direction finding

Magnetic Variation

• Variation is the difference between the True and Magnetic north in degrees
• Expressed as magnetic north being east or west of true north
• Variation dependent on the relative position of the Magnetic north and Aircraft
• Maximum value of variation can be 180 deg east or west
• Isogonal are lines joining places of equal variation
• Converge on the True and Magnetic poles
• Agonal are lines joining places zero variation

Magnetic Variation

• Plus and Minus signs are applied to magnetic heading to get true heading
• Magnetic direction can be calculated from variation
• Variation west (-) magnetic best
• Variation east (+) magnetic least

Magnetic Dip

• Magnetic field can be resolved into horizontal & vertical components
• Horizontal component is the directive force to determine magnetic north
• Magnetic dip is the angle between the earth’s magnetic field from horizontal
• Dip angle is zero at equator, magnet is horizontal to surface of earth
• Dip angle is 90 degrees at poles. magnet is vertical to earth’s surface

Effect of Magnetic Dip

• Lines of Inclination join places of equal magnetic dip
• Aclinical line joins places of zero dip angle seen at the magnetic equator
• Field strength is the measure of directive force in micro teslas
• Compass requires a minimum of 6 micro teslas to function properly

Compass Deviation

• Electric and metallic objects near aircraft compass cause error called deviation
• Compass deviation is difference between the magnetic & compass north
• Expressed in degrees east or west of magnetic north

Calculation of Compass Deviation

• Compass swing is the procedure to prepare a compass deviation card
• Plus and Minus signs are applied to compass heading to get magnetic heading
• Compass direction can be calculated from magnetic direction and deviation
• Deviation west (-) compass best
• Deviation east (+) compass least

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